In the recent years, aluminum and aluminum alloy (referred to an aluminum herein after) have been used widely for interior materials of buildings, various vehicles and transportation machines. Welded joints of these aluminum used in the above fields form directly an outside appearance and hence are required to have a beautiful appearance of the welding beads in addition to a sufficient mechanical strength. As an arc welding method showing a beautiful appearance of welding beads, a widely used method is a TIG arc welding method having a filler wire added thereto. The welded joint obtained with a TIG arc welding having the filler wire added thereto (referred to a TIG filler arc welding method hereinafter) shows a bead appearance in regular wave form (referred to a scale bead hereinafter) as shown in FIG. 1. The welding bead according to the TIG filler arc welding method has an appearance more beautiful than that of the MIG arc welding method.
The TIG arc welding method is lower in the welding speed than that of MIG arc welding method which melts the consumable electrode and is in a lower production efficiency. Therefore, various proposals have been conducted to a method in which the MIG arc welding method could be able to achieve the welding bead appearance near to the scale bead due to the TIG arc welding method having the filler wire added thereto.
The prior art 1 is a MIG arc welding method disclosed in the Japanese Patent Publication (examined) Syo 46-650 and is to change alternatively the arc spray amount between the high value and the low value. In order to actually execute this method, the electric power supplied to the arc is alternatively changed between the relatively high value and relatively low value (for example 3:2).
FIG. 2 shows a block diagram of an electric source for use in this welding method. FIG. 3 is a graph showing a V-I characteristic between an output current (horizontal axis) and an output voltage (vertical axis) (referred to V-I characteristic hereinafter) and an arc characteristic. With reference to FIG. 2, a reference numeral 101 denotes an output circuit for welding electric source; a reference numeral 102, a resistor for switching the V-I characteristic of the output circuit for welding electric circuit 101; a reference numeral 103, a contact for shortcircuiting and opening the resistor 101; and a reference numeral 104, a timer for switching periodically the contact 103. A reference numeral 1 denotes a consumable electrode wire (referred to a consumable electrode hereinafter) transferred to a welding material 2 through an electric supplying tip 4 at a predetermined speed by a wire supplying motor WM. An arc 3 is generated between the consumable electrode 1 and the welding material 2 and is 10 to weld the welding material 2.
FIG. 3 shows a V-I characteristic AA applied between the consumable electrode 1 and the welding material 2 when the resistor 102 is connected to the output circuit, a V-I characteristic BB when the resistor 102 is short circuited and an arc characteristic at arc lengths of L1 to L3 shown by dotted lines L1 to L3. It is assumed that an operation point is a crossing point A of a solid line AA and a dotted line L1 of FIG. 3 when the consumable electrode 1 is transferred at predetermined value of wire supplying speed WF1. At this time, a welding electric current is in I1 and an arc voltage is in V1.
There are three methods for switching the above electric power.
(1) A first method is to switch an output voltage supplied across the consumable electrode 1 and the welding material 2. For example, in FIG. 3, when the V-I characteristic is switched from AA to BB, the welding electric current I1 does not change but the operation point moves usually to B point of the V-I characteristic curve BB because the wire supplying speed and the welding electric current are not proportional perfectly to each other. However, at the transient time, the arc length L1 does not rapidly change with the rapid change in the V-I characteristic curve into the BB curve. Therefore, the operation point moves from a point A on the arc characteristic curve L1 the same as the above curve to a point D. The movement causes an increase in the welding electric current and then an increase in the wire melting speed which results in an increase in the arc length. Then, the arc length changes from the curve L1 to an arc characteristic curve L2 having a long arc length and finally moves to a point B. Therefore, the welding current changes largely from I1 to I3 at a transient state and from I1 to I2 at a stationary state.
(2) A second method is to switch the wire supplying speed. For example, in FIG. 3, when the wire supplying speed is switched, the switching is carried out so as to change the arc length on the V-I characteristic AA and hence the operation point changes from a point B to a point C. Accordingly, the welding electric current changes largely from I2 to I4 and the arc length moves from the operation point of the arc characteristic curve L2 to the operation point of the curve L3.
(3) A third method is to change the V-I characteristic of above case (1) and the wire supplying speed at case (2) simultaneously. Accordingly, the welding electric current changes largely from 11 to I4.
The prior art 2 is a MAG welding method described in Japanese Patent Publication (examined) Sho 49-48057 and is to supply the consumable electrode at a predetermined speed by using a constant voltage characteristic of a welding electric source. The welding method according to prior art 2 is to change periodically the welding electric current and simultaneously changing the welding wire supplying speed with a change in the welding electric current. That is, this method is to switch between a high electric current (a high output voltage) due to a high speed of wire supplying and a low current (a low output voltage) due to a low speed of wire supplying and to control input heat. FIG. 4(A) to (C) are graphs showing V-I characteristic curves AA and BB between the output current of the welding electric source (horizontal axis) and the output voltage (vertical axis) and arc characteristic curves L1 and L2.
There are three practical methods for controlling the input heat mentioned above.
(1) A first method is not to change the arc length at both of the high electric current and the low electric current. As shown in FIG. 4(A), for example, when the both of the V-I characteristic of the welding electric source and the wire supplying speed are switched, the operation point moves from the operation point A at the crossing point of the V-I characteristic curve AA and the arc characteristic curve L1 to the operation point B of the crossing point of the V-I characteristic curve BB and the arc characteristic curve L1. At this time, the welding current changes largely from I1 to I2 while the arc length is L1 on the same curve does not change.
(2) A second method is to change in a larger degree the wire supplying speed than the case (1) when the arc length does not change. That is, with a high electric current, the wire supplying speed is made higher and the arc length is made shorter while with a low electric current, the wire supplying speed is made low and the arc length is longer. For example, as shown in FIG. 4(B), when the wire supplying speed is switched, the operation point moves from the crossing point A of the V-I characteristic curve AA and the arc characteristic curve L2 to the crossing point B of the V-I characteristic curve AA and the arc characteristic curve L1. In this time, the welding current changes largely from I1 to I2 while the arc length changes from L2 to L1.
(3) A third method is to change the wire supplying speed in a smaller degree than the case (1) where the arc length does not change. At the high electric current, the wire supplying speed is made low and the arc length is made long while at the low electric current, the wire supplying speed is made high and the arc length is made short. For example, as shown in FIG. 4(C), when both of the V-I characteristic of the welding electric source and the wire supplying speed are switched, the operation point at the stationary state moves from the crossing point A of the V-I characteristic curve AA and the arc characteristic curve L1 to a crossing point B of the V-I characteristic curve BB and the arc characteristic curve L2. However, at the transient state, even when the V-I characteristic curve changes rapidly into the curve BB, the arc length does not rapidly change. However, the operation point moves from the point A to the point D with the same arc length to each other, while the welding electric current increases. As a result, the wire melting speed is also higher and the arc length is longer. The arc length moves from the curve L1 to the curve L2 and finally the operation point moves to the point B. Therefore, the welding electric current changes largely from I1 to I3 at the transient state and from I1 to I2 at the stationary state.
The prior art 3 is a pulse MIG welding method described in the Japanese Patent Publication (unexamined) Syo 62-279087 and is to switch a base current or base voltage switch between a spray transfer and a short circuit transfer to form a scale bead without melting down aluminum thin plate.
(1) For example, as shown with a wave form of a welding electric current in FIG. 5(A), the use of a pulse welding electric source comprising a constant pulse current in which either of pulse current, pulse width or pulse frequency is a constant value and a variable base current permits the spray transfer and the short circuit transfer to be carried out alternatively by changing the base current periodically in a large degree.
(2) As shown with a wave form of a welding voltage in FIG. 5(B), the use of a pulse welding electric source outputting a welding voltage composed of a pulse voltage in which either of a pulse voltage, pulse width or pulse frequency is a constant value and variable base voltage permits the spray transfer and the short circuit transfer to be carried out alternatively by changing the base voltage periodically in a large degree.
Accordingly, this prior art 3 switches the wire feed rate as shown in FIG. 5(A) and hence changes periodically the average value of welding current in a large scale by changing the welding current values into an average value H1 at the high current period T8 and the average value N1 at the low current period T9. Thus, it is possible to switch between the spray transfer and the short circuit transfer. Further this prior art 3 switches the output power of the base electric source in a large scale as shown in FIG. 5(B). In the method shown in FIG. 5(B), the welding current value changes in a small scale at the stationary state but changes in a large scale at the transient state in a similar way to that of the prior art 1.